MediaWiki:Apihelp-query+categorymembers-param-endsortkeyprefix
Sortkey prefix to end listing before (not at; if this value occurs it will not be included!). Can only be used with $1sort=sortkey. Overrides $1endhexsortkey.
Sortkey prefix to end listing before (not at; if this value occurs it will not be included!). Can only be used with $1sort=sortkey. Overrides $1endhexsortkey.